941 research outputs found
Violating the string winding number maximally in Anti-de Sitter space
We study n-string scattering amplitudes in three-dimensional Anti-de Sitter
space (AdS3). We focus our attention on the processes in which the winding
number conservation is violated maximally; that is, those processes in which it
is violated in n-2 units. A worldsheet conformal field theory calculation leads
us to confirm a previous conjecture about the functional form of these
observables.Comment: 15 pages. V2 references added and minor typo corrected. V3, V4
addendum on an alternative derivation of the main formula for short string
Ferroelectric Dynamics in the Perovskite Relaxor PMN
We review results obtained from recent neutron scattering studies of the
lead-oxide class of perovskite relaxors PMN and PZN. A ferroelectric soft mode
has been identified in PMN at 1100 K that becomes overdamped near 620 K. This
is the same temperature at which polar nanoregions (PNR) begin to form, denoted
by Td, and suggests that a direct connection exists between the soft mode and
the PNR. The appearance of diffuse scattering intensity at Td reported by
Naberezhnov et al. lends further support to this picture. At lower temperature
the soft mode in PMN reappears close to Tc = 213 K (defined only for E > Ec).
These results are provocative because the dynamics below Tc are characteristic
of an ordered ferroelectric state, yet they occur in a system that remains
cubic on average at all temperatures. We discuss a coupled-mode model that
successfully describes these data as well as those from earlier lattice
dynamical studies of other perovskites such as BaTiO3.Comment: 10 pages, 7 figures, Conference - Fundamental Physics of
Ferroelectrics 200
Systematic Study of Short Range Antiferromagnetic Order and The Spin-Glass State in Lightly Doped La2-xSrxCuO4
Systematic measurements of the magnetic susceptibility were performed on
single crystals of lightly doped La2-xSrxCuO4 (x=0.03, 0.04 and 0.05). For all
samples the temperature dependence of the in-plane magnetic susceptibility
shows typical spin-glass features with spin-glass transition temperatures Tg of
6.3K, 5.5K and 5.0K for x=0.03, 0.04 and 0.05, respectively. The canonical
spin-glass order parameter extracted from the in-plane susceptibility of all
the samples follows a universal scaling curve. On the other hand, the
out-of-plane magnetic susceptibility deviates from Curie law below a
temperature Tdv, higher than Tg. Comparing with previous neutron scattering
results with an instrumental energy resolution of 0.25 meV from Wakimoto et
al., the x-dependence of Tdv is qualitatively the same as that of Tel, the
temperature below which the elastic magnetic scattering develops around (pi,
pi). Thus, a revised magnetic phase diagram in the lightly doped region of
La2-xSrxCuO4 is proposed. The Curie constants calculated from the in-plane
susceptibility are independent of the Sr concentration. On the basis of the
cluster spin-glass model, this fact might reflect an inhomogeneous distribution
of doped holes in the CuO2 plane, such as in a stripe structure.Comment: 7 pages, 6 figure
Inelastic neutron scattering study on the resonance mode in an optimally doped superconductor LaFeAsOF
An optimally doped iron-based superconductor LaFeAsOF with
K has been studied by inelastic powder neutron scattering. The
magnetic excitation at \AA is enhanced below , leading to
a peak at meV as the resonance mode, in addition to the
formation of a gap at low energy below the crossover energy . The peak energy at \AA corresponds to in
good agreement with the other values of resonance mode observed in the various
iron-based superconductors, even in the high- cuprates. Although the
phonon density of states has a peak at the same energy as the resonance mode in
the present superconductor, the -dependence is consistent with the resonance
being of predominately magnetic origin.Comment: 4 pages, 5 Postscript figure
Static magnetic correlations near the insulating-superconducting phase boundary in LaSrCuO
An elastic neutron scattering study has been performed on several single
crystals of LaSrCuO for {\it x} near the lower critical
concentration {\it x} for superconductivity. % In the insulating
spin-glass phase ({\it x} = 0.04 and 0.053), the previously reported
one-dimensional spin modulation along the orthorhombic {\it b}-axis is
confirmed. % Just inside the superconducting phase ({\it x} = 0.06), however,
two pairs of incommensurate magnetic peaks are additionally observed
corresponding to the spin modulation parallel to the tetragonal axes. % These
two types of spin modulations with similar incommensurabilities coexist near
the boundary. % The peak-width along spin-modulation direction
exhibits an anomalous maximum in the superconducting phase near {\it x},
where the incommensurability monotonically increases upon doping
across the phase boundary. % These results are discussed in connection with the
doping-induced superconducting phase transition.Comment: 9pages, 9figure
Neutron-scattering study of spin correlations in La1.94-xSrxCe0.06CuO4
We performed a neutron-scattering experiment to investigate the effect of
distortion of CuO2 planes on the low-energy spin correlation of
La1.94-xSrxCe0.06CuO4 (LSCCO). Due to the carrier-compensation effect by
co-doping of Sr and Ce, LSCCO has a smaller orthorhombic lattice distortion
compared to La2-xSrxCuO4 (LSCO) with comparable hole concentration p. A clear
gap with the edge-energy of 6~7 meV was observed in the energy spectrum of
local dynamical susceptibility c"(w) for both x=0.18 (p~0.14) and x=0.24
(p~0.20) samples as observed for optimally-doped LSCO (x=0.15~0.18). For the
x=0.14 (p~0.10) sample, in addition to the gap-like structure in c"(w) we
observed a low-energy component within the gap which develops below 2~3meV with
decreasing the energy. The low-energy component possibly coincides with the
static magnetic correlation observed in this sample. These results are
discussed from a view point of relationship between the stability of low-energy
spin fluctuations and the distortion of CuO2 planes.Comment: 4 pages, 3 figures, proceeding for SNS2007 conferenc
Diagonal static spin correlation in the low temperature orthorhombic Pccn phase of La1.55Nd0.4Sr0.05CuO4
Elastic neutron scattering measurements have been performed on Nd, Sr
co-doped La1.55Nd0.4Sr0.05CuO4, which exhibits a structural phase transition at
Ts ~ 60K from the low temperature orthorhombic Bmab phase (labelled LTO1) to
the low temperature orthorhombic Pccn phase (labelled LTO2). At low
temperatures, well below Ts, elastic magnetic peaks are observed at the
``diagonal'' incommensurate (IC) positions (0, 1+/-0.055, 0), with modulation
direction only along the orthorhombic b-axis just as in Nd-free
La1.95Sr0.05CuO4. In the present study, the one-dimensionality of the IC
modulation, which is naturally explained by a stripe model, is clearly
demonstrated with our "single-domain" crystal. The temperature dependence of
the IC peak intensity suggests a substantial contribution from the Nd3+ spins
below ~3K. Consistent with this, the L dependence of the magnetic scattering is
accurately accounted for by a model in which the contribution of the Nd3+ spins
is explicitly included.Comment: 8 pages, 8 figure
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